A laser is a device that uses light flashes to excite the atoms

[garytse86]

A laser is a device that uses light flashes to excite the atoms so you get electrons with energy levels about 2 or 3 levels above the ground state. They emit photons, which vibrate at a particular frequency, they get reflected off the half silvered mirror, to excite electrons and make them emit pohtons at the same frequency. So the wavelength of the laser is just one, nothing else, but surely an atom has got more than one electron in the outer shell, wouldn't this cause a problem because either the atom would emit photons with different frequencies, because two particles in the same atom cannot be at the same state, according to the exclusion principle.

 

[mathman]

There are two issues here.
The second, having to do with the exclusion principle, is irrelevant, since each atom has only one electron in the upper state. The laser works because there are a lot of atoms, in the medium of interest, which have been excited.
The first issue, about possible other excited states, is a function of how the excitation takes place. Lasers are designed so that most of the energy, used in the excitation, is tailored to the particlular level desired.

 

[Claude Bile]

A laser works by establishing a population inversion in some gain material. The process of creating a population inversion is called pumping. Lasers can be pumped with bright flashlamps, other lasers, chemicals, electricity etc.

When a population inversion is established, the material gives an optical gain. This is due to the process of stimulated emission resulting in an avalanche effect. Feedback increases this effect.

Lasers do not emit at a distinct frequency, the emission spectrum is broadened due to the uncertainty principle and the finite lifetime of the upper state of the atoms in the gain medium. Typical broadening is about 8 MHz.

Two electron systems do exhibit some energy splitting on their own, however in a medium with lots of atoms, energy levels broaden into energy bands. This manifests itself as the spectrum broadening mention above.